On March 11, 1998, a Bell 47D1 (registration N13012) was involved in an aviation accident near Jeanerette, LA. Investigators recorded the probable cause as: The in-flight disconnect of the rotating scissors due to the separation of its hinge bolt. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
While practicing landings, a helicopter encountered severe vibrations and made a hard landing, rolling onto its left side. An inspection revealed a missing scissor hinge bolt.
Accident Sequence
A dual student pilot was practicing landings in a local practice area and had completed four to five landings. On the subsequent landing, the helicopter was heading west at 350 feet above ground level with an airspeed of 35 mph. As the student lowered the collective to initiate a steep approach, a severe vertical vibration occurred. Approximately two to three seconds later, a severe lateral vibration began. The flight instructor then took the controls and continued the descent toward the landing area. At about 10 feet, the instructor attempted to reduce forward airspeed and descent rate by moving the cyclic aft and raising the collective. According to the instructor, 'Neither of those inputs seemed to help a lot and we impacted hard.' The helicopter rolled onto its left side before coming to rest heading east. No issues with tail rotor control were reported.
Inspection Findings
An examination of the helicopter by a Federal Aviation Administration (FAA) inspector revealed that the scissor hinge bolt, which attaches the scissor to the swash plate, was missing. Maintenance records indicated that the helicopter had undergone a 100-hour inspection on February 15, 1998, approximately 20 flight hours prior to the accident.